1. Field of the Invention
The invention relates to a biologically inspired pitching and flapping mechanism for small-scale flight. The mechanism allows for a micro aircraft to harness some of the unsteady aerodynamic mechanisms that can be found in biological small-scale flyers, such as insects and small birds.
2. Description of the Related Art
Over the last several years, the Micro Air Vehicle (MAV) program sponsored by the Defense Advanced Research Projects Agency (DARPA) developed a series of successful fixed wing aircrafts, such as the Black Widow (Aerovironment) and the Microstar (Lockheed Martin). These vehicles demonstrated satisfactory performance for a limited type of reconnaissance mission. As specified by DARPA's initial requirements, all the vehicles have dimensions no larger than six inches and weigh between 80 and 110 grams. The vehicles can fly for approximately 30 minutes while carrying about 20 grams of payload. The size constraints of the vehicles make them one order of magnitude smaller than previously developed systems. As such, the vehicles are required to operate under low Reynolds number aerodynamic flow conditions (between 5,000 and 50,000Re). The applicability of the fixed wing MAVs is greatly reduced by inadequate performance characteristics such as the lack of hover capability, poor maneuverability in tightly constrained environments, and the inability for multiple takeoffs and landings during a single mission. These performance characteristics become important when a specific mission requires the surveillance of a target from a fixed location, for example, from the top of a building in an urban environment. In such a mission, it would be necessary for the vehicle, after reaching the target, to land without interrupting data transfer, thereby saving energy and increasing the system's versatility and potential mission duration. If the target moves, the vehicle could take off and relocate to a new position. Rotary wing vehicles fill the gap left by their fixed wing counterparts, offering the necessary characteristics required for these types of missions. Nevertheless, hovering capability comes at a price, since the energy requirements of a rotorcraft in hover are almost double that of a fixed wing aircraft of similar weight in cruise. The University of Maryland's Alfred Gessow Rotorcraft Center developed a MIcro COaxial Rotorcraft (MICOR), which is an electrically driven, battery powered coaxial helicopter weighing about 100 grams. See FIG. 1. The six-inch diameter three bladed rotors use 8% cambered curved plate airfoils and have a solidity of 0.1068. Rotor performance was evaluated and a maximum hover efficiency or Figure of Merit (FM) of 42% was obtained using a 10% linear twist in the blades.
The aerodynamic performance of the MICOR rotors was poor considering that full-sized helicopters have an FM between 70 and 85%. The modest FM values obtained can be explained by the low Reynolds number flow conditions that the rotors encounter (approximately 30,000 in hover). Under these flow conditions the viscous forces dominate over the inertial forces, reducing the maximum lift coefficient and greatly increasing the profile drag, and thus, power consumption. The reduced airfoil performance and the limited power density of the batteries yielded a hover endurance of only 3 minutes. Under low Reynolds number flow conditions there is little that can be done to improve the efficiency of a conventional rotor, because all airfoils will suffer similar performance deterioration. Other sources of inefficiency such as tip losses and wake swirl are marginally reduced by optimizing the rotor's main design parameters (solidity, number of blades, blades shape etc.). Thus, there is a need to provide a rotary wing aircraft that has improved airfoil performance, hovering capabilities and maneuverability in order to perform complex surveillance.
The inventors of the present invention have observed that at low Reynolds numbers, such as 30,000 and below, flying insects and some small birds have remarkably high lift forces. Wind tunnel tests and analytical models of insect wings show that under a steady flow with no wing actuation, aerodynamic forces are smaller than those required for active flight. Complex animal wing motion can be categorized into three main basic movements: flapping, pitching and translation. Resultant wing movement produces a series of lift enhancing aerodynamic mechanisms: delayed stall, rotational circulation and wake capture. While aerodynamic mechanisms such as wake capture and rotational lift have been observed in insects' flight, few if any manmade miniature flying machines make any attempt at harnessing these mechanisms. Two exceptions to this include Caltech's Microbat and U.C. Berkeley's micro-robotic flying device.
The Microbat is an ornithopter type vehicle, in that it has flapping wings to generate the required lift and thrust. The Microbat uses a lightweight, low-friction transmission mechanism to convert the rotary motion of a driving electric motor into the flapping motion of the wings. The transmission design restricts the flapping motion in a plane perpendicular to the motor shaft. A small DC motor is used to drive the transmission. The Microbat transmission design can only be used in an ornithopter configuration, and has only one degree of freedom, specifically, the wings only flap. The dimensions of the mechanism determine the amplitude of the flapping and the amplitude is fixed for a given prototype. The Microbat further lacks the rotational and pitching degrees of freedom as well as the variable amplitude of the movements of the present invention.
The U.C. Berkeley micro-robotic flying device uses separate four-bar frames to control the leading and trailing edges of a fanfold compliant wing. The device works exclusively in an ornithopter configuration. Actuation of the device is performed by the use of a piezoelectric unimorph with small angle deflection. The device provides pitching and flapping degrees of freedom to a flexible wing, however the pitching and the flapping are coupled. Thus, a rigid wing cannot be used, since it would over-constrain the device.
Insects use various types of unsteady mechanisms depending on the different flow conditions their wings encounter. A typical wing beat cycle can roughly be divided into four different stages: downstroke, supination, upstroke, and pronation. These four stages are illustrated in the FIG. 2 diagram of the vortex system during the complete wingbeat cycle. (H. Liu, et al. “A Computational Fluid Dynamic Study of Hawkmoth Hovering,” J. of Exp. Bio, 201, 461-477, 1998). The shaded area at pronation denotes the morphological lower wing surface on the insect diagram (insets). A large leading-edge vortex (LEV) with strong axial flow is observed during the downstroke. This LEV is still present during supination, but turns into a hook-shaped vortex. A small LEV is also detected during the early upstroke, and gradually grows into a large vortex in the latter half of the upstroke. This LEV is still observed closely attached to the wing during the subsequent pronation, where a trailing-edge vortex (TEV) and a shear-layer vortex (SLV) are also formed, together forming a complicated vortex system. Downstroke and upstroke are translational movements, where the wings advance through the air at high angles of attack. Supination and pronation correspond to rapid rotational movements at the end of the downstroke and upstroke respectively. The wing rotation inverts the upper and lower surface of the wing keeping a positive angle of attack during the translational phases. There are three main unsteady aerodynamic effects: delayed stall, rotational circulation, and wake capture. These effects are briefly described below.
Delayed stall can be similar to the dynamic stall found in full size helicopters. In a delayed stall, the blades in the retreating side encounter high angles of attack and when the airfoil exceeds the stall angle, a leading edge vortex is formed. This vortical disturbance increases the suction pressure and thus, the overall lift of the wing as long as it remains attached to the upper surface. However the pitching moment and the lift-to-drag ratio are negatively affected. As soon as the vortex is created, it starts convecting downstream towards the trailing edge until it is shed, leaving a separated flow on the airfoil and canceling all additional lift production. The angle of attack at which reattachment occurs is usually below the static stall angle so the phenomenon exhibits hysteresis. The overall effect of dynamic stall on the rotor's performance depends on several factors including reduced frequency, Mach number, mean angle of attack, and airfoil design. During the flight of an insect, when dynamic stall occurs and the leading edge vortex is created, an axial flow on the wing stabilizes the vortex, reducing the convection speed towards the trailing edge and thus allowing additional lift production through the entire downstroke and upstroke. The axial flow on the wing is produced by the pressure gradient between the root and the tip of the wing due to the flapping and lead-lag movements. Due to the extended time the vortex stays attached to the airfoil, the phenomenon is referred to as delayed stall.
Rotational circulation and wake capture are unsteady mechanisms associated with the rotational portions of the wingbeat cycle. On a rotating cylinder, the pressure on the high velocity side will be lower than the pressure on the low velocity side. Hence, a pressure difference exists, causing a side thrust or lift on the cylinder. However, insect wings are not circular in cross-section, so the resultant lift is perpendicular to the surface during the rotation. Insects rotate their wings just before stroke reversal occurs in order to create an upward force because lift orientation is dependent upon the relative direction of the incoming flow and the wing's rotation. Just after stroke reversal and before any significant translational speed is reached, an additional transient lift production has been identified. This phenomenon is known as wake capture, in which the wings use the shed vorticity of the previous stroke to enhance its lift. The flow generated by a translational phase, either upstroke or downstroke, increases the effective fluid velocity of the next one, raising the thrust levels above the ones found in translation alone.
In order to harness the unsteady lift mechanisms used by most insects, the inventors of the present invention have developed a biologically inspired flapping/pitching mechanism in conjunction with the rotary wing concept. This mechanism replicates some of the aerodynamic phenomena that enhance the performance of small fliers, replacing the periodic translational motion with a unidirectional circular motion while actively flapping and pitching the rotor blades. The present invention is novel as it is directed to a rotating rotor that can undergo large-scale pitching and flapping motions at high frequencies. Further, the present invention uses some of the aforementioned unsteady phenomena in a pitching and flapping mechanism to improve the aerodynamic performance of a small-scale rotary wing aircraft.